Bromide substitution in lithium borohydride, LiBH4-LiBr

By means of in situ synchrotron radiation powder X-ray diffraction, powder neutron diffraction, attenuated total reflectance infrared spectroscopy, differential scanning calorimetry and the Sieverts techniques we have investigated how anion substitution in the LiBH4-LiBr system leads to changes in the structural, physical, chemical and hydrogen storage properties of this material. Mechano-chemical treatment facilitates formation of a hexagonal solid solution h-Li(BH4)(1-x)Br-x whereas heating at elevated temperatures, i.e. T > 112 degrees C appears to allow full solubility in the system LiBH4-LiBr. The first step in the anion substitution process may be dissolution of small amounts of LiBH4 in alpha-LiBr deduced from observation of a hexagonal solid solution with unit cell volume similar to beta-LiBr for a hand-mixed sample. The solid solution, Li(BH4)(1-x)Br-x, is isostructural to the hexagonal high temperature polymorph of LiBH4. This solid solution melts at a significantly higher temperature depending on the composition as compared to h-LiBH4. Furthermore, a new hexalithium borate tribromide, Li-6(BO3)Br-3 was discovered and structurally characterized.